1. Field of the Invention
The present invention relates to a method for producing colored resin particles, colored resin particles produced by this method, a developer containing the colored resin particles, an image forming apparatus, an image forming method, and a process cartridge each using the developer.
2. Description of the Related Art
In electrophotographic image forming apparatuses, colored resin particles containing a colorant are used as a toner to form visible images. Colored resin particles are also used for image formation of electronic paper.
Previously proposed methods for producing colored resin particles used as a toner include: a method in which a binder resin such as a polyester resin, a colorant, a releasing agent, etc. are dissolved or dispersed in an organic solvent and the obtained oil phase is dispersed in an aqueous phase to produce colored resin particles (the dissolution suspension method); and a method in which fine polyester resin particles, fine pigment particles, fine releasing agent particles, etc. are aggregated in the presence of, for example, an aggregating salt for adjusting their shape to thereby produce colored resin particles (the emulsification association method) (see Japanese Patent Application Laid-Open (JP-A) No. 63-25664 and Japanese Patent Application Publication (JP-B) No. 61-28688).
Colored resin particles used for electrophotography and electronic paper preferably have a uniform particle diameter. The dissolution suspension method is suitably used for producing such colored resin particles that have a uniform particle diameter.
In view of this, as a method for producing particles having a uniform particle diameter by the dissolution suspension method, there has been proposed a method in which fine resin particles are dispersed in an aqueous phase and utilized to stabilize dispersion of liquid droplets of an oil phase (see Japanese Patent (JP-B) Nos. 3455523 and 4134057 and JP-A No. 02-43225). Specifically, there are proposed a method for producing resin particles in which liquid droplets of a resin solution (oil phase) are dispersed in an aqueous medium containing fine resin particles dispersed therein whereby dispersion of the oil phase liquid droplets are stabilized, and resin particles obtained by the method.
However, the dispersion stability of the oil droplets in these proposed techniques depends on, for example, properties as dispersion particles of fine resin particles such as particle diameter and zeta potential. In order to stably produce colored resin particles, it is necessary to strictly manage fine resin particles according to their particle diameter and zeta potential. Therefore, they have problems in production stability.
There has also been proposed a method in which an oil phase is dispersed in an aqueous phase to form oil droplets, and a dispersion liquid of fine vinyl resin particles is added to the obtained oil droplets dispersion liquid before or after removal of the solvent therefrom, followed by heating, to make the vinyl resin particles exist on the oil droplets or the particles from which the solvent has been removed (see JP-A No. 2006-285188).
This proposed technique involves heating at a high temperature of 70° C. or higher for surely attaching the vinyl resin particles onto the particles as cores, requiring a large amount of energy. Thus, this method is not a preferred method from an economical viewpoint and in consideration of environmental load.
When a vinyl resin having less polar groups such as a carboxyl group is added before removal of the solvent, oil droplets decrease in dispersion stability to cause aggregation or agglomeration between the oil droplets, raising a problem that colored resin particles having a uniform particle diameter cannot stably be produced.
At present, demand has arisen for development of a method for producing colored resin particles by the dissolution suspension method, which can stably produce colored resin particles having a uniform particle diameter and can require less energy for the production, colored resin particles produced by this method, a developer containing the colored resin particles, and an image forming apparatus, an image forming method, and a process cartridge each using the developer.